The present invention relates to an image sensor used as a reading unit for facsimile machines or image scanners, and more particularly to an image sensor having additional capacitors for temporarily storing electric charges that are generated at photoelectric conversion elements by photoelectric converting operation, each of such additional capacitors being small in surface area and large in capacitance.
Among conventional image sensors, especially of a contact type, there is a TFT-driven image sensor which employs thin film transistor switching elements (TFTs . In the TFT-driven image sensor, image data such as a document is projected on a one-to-one correspondence basis and the projected image data is converted into electric signals. In this case, the projected image is divided into a multiplicity of pixels (photoelectric conversion elements) and electric charges generated at the respective photoelectric conversion elements are temporarily stored at load capacitors disposed in multilayer interconnections or predetermined blocks using thin film transistor switching elements. Then, the stored electric charges are sequentially read as electric signals at a speed within the range from several hundreds of KHz to several MHz. Such a TFT-driven image sensor can read image data with a single drive IC owing to TFT operation, thereby contributing to reducing the number of drive ICs that drive the image sensor.
As shown by, e.g., an equivalent circuit in FIG. 7, this TFT image sensor includes: a line-like photoelectric conversion element array 11 whose length is substantially the same as the length of a document; an electric charge transfer section 12 consisting of a plurality of thin film transistors Ti,j (i=1 to N, j=1 to n) corresponding to respective photoelectric conversion elements 11' on a one-to-one basis; and multilayer interconnections 13.
The photoelectric conversion element array 11 consists of N blocks of photoelectric conversion elements 11'. Further, n photoelectric conversion elements 11', which constitute each block, can be represented equivalently as a plurality of pairs of photodiodes PDi,j (i=1 to N, j=1 to n) and parasitic capacitors CDi,j (i=1 to N, j=1 to n). Each photoelectric conversion element 11' is connected to the drain electrode of each thin film transistor Ti,j. The source electrode of each thin film transistor Ti,j is connected to one of n common signal lines 14 and to one of the load capacitors CLi (i=1 to n) accompanying a respective photoelectric conversion element through the matrix-like multilayer interconnections 13. Further, each common signal line 14 is connected to a drive IC 15.
The gate electrode of each thin film transistor of a given block is commonly connected to a gate pulse generating circuit (not shown) so that the thin film transistors Ti,j of the given block can be turned on simultaneously. The photoelectric charge generated at each photoelectric conversion element 11' is temporarily stored at both the parasitic capacitor CDi,j in each photoelectric conversion element and an overlap capacitor existing between the drain and gate of each thin film transistor and subsequently transferred to and stored in a corresponding load capacitor CLi disposed in the multilayer interconnections 13 of a respective block using a thin film transistor Ti,j as an electric charge transfer switch.
Specifically, a gate pulse .phi.G1 from the gate pulse generating circuit turns on the thin film transistors T1,1 to T1,n of a first block, thereby causing the electric charge generated at each photoelectric conversion element 11' and stored at each parasitic capacitor CDi,j and the like in the first block to be transferred to and stored at each respective load capacitor CLi. The electric charge stored at each load capacitor CLi changes the potential of each corresponding common signal line 14, and each changed potential is received at an output line 16 by sequentially turning on an analog switch SWi (i=1 to n) within the drive IC 15.
Then, the thin film transistors T2,1-T2,n to TN,1-TN,n of a second to N-th blocks are similarly turned on by gate pulses .phi.G2 to .phi.Gn, transferring the electric charges from the photoelectric conversion elements of each block. By sequentially reading the transferred electric charges, image signals equivalent to a single line in a main scanning direction of the document can be obtained. The above operation is repeated in connection with the document moved by document feeding means (not shown) such as rollers, thereby allowing the image signals of the entire document to be obtained (see Japanese Patent Unexamined Publications Nos. Sho. 63-9358 and Sho. 63-67772).
Specific structures of the photoelectric conversion element and TFT for use in the above-described conventional image sensor will be described with reference to FIG. 8 which is a plan view illustrating the photoelectric conversion element and TFT and FIG. 9 which is a sectional view illustrating a portion taken along a line B-B' shown in FIG. 8.
As shown in FIGS. 8 and 9, the conventional photoelectric conversion element is of a sandwiched structure having a belt-like metal electrode 22 made of, e.g., Cr that acts as a lower common electrode, a photoconductive layer 23 made of amorphous silicon hydride (a-Si:H) that is segmented into each photoelectric conversion element 11' (i.e., each bit), and a similarly segmented upper transparent electrode 24 made of indium-tin oxide (ITO) deposited on an insulating substrate 21 made of, e.g., glass or ceramics.
The lower metal electrode 22 is formed so as to extend belt-like in the main scanning direction, and the photoconductive layer 23 is sparsely segmented on the metal electrode 22 while the upper transparent electrode 24 is similarly sparsely segmented so as to form individual electrodes. As a result, a portion interposing the photoconductive layer 23 between the metal electrode 22 and the transparent electrode 24 constitutes a photoelectric conversion element 11', a group of such portions constituting the photoelectric conversion element array 11. A predetermined voltage VB is applied to the metal electrode 22.
An end of a line 30a made of, e.g., Al is connected to an end of each sparsely segmented transparent electrode 24, while the other end of the line 30a is connected to the drain electrode 41 of a corresponding thin film transistor Ti,j of the electric charge transfer section 12.
As shown in FIGS. 8 and 9, the TFT for use in the conventional image sensor has a reverse staggered structure. Specifically, the TFT is formed by sequentially depositing, on the substrate 21, a chromium (Crl) layer serving as a gate electrode 25, a silicon nitride (SiNx) film serving as a gate insulating layer 26, an amorphous silicon hydride (a-Si:H) layer serving as a semiconductor activated layer 27, a silicon nitride (SiNx) film serving as a top insulating layer 29 that is arranged so as to confront the gate electrode 25, an n.sup.+ amorphous silicon hydride (n.sup.+ a-Si:H) layer serving as an ohmic contact layer 28, and a chromium (Cr2) layer serving as a drain electrode 41 and a source electrode 42, with additional depositions of a polyimide insulating layer on the Cr2 layer, and of the line layer 30a on the polyimide layer or an Al layer 30 above the top insulating layer 29 to shield the a-Si:H layer.
The Al layer 30 for shielding the a-Si:H layer is provided to prevent light from provoking photoelectric conversion caused by allowing the light to transmit through the top insulating layer 29 and inject into the a-Si:H layer. The line 30a from the transparent electrode 24 in the photoelectric conversion element 11' is connected to the drain electrode 41. The ohmic contact layer 28 is separated into a partial layer 28a that is in contact with the drain electrode 41 and a partial layer 28b that is in contact with the source electrode 42, and the Cr2 layer serving as the drain electrode 41. The source electrode 42 is similarly separated so as to cover the ohmic contact layer portions 28a and 28b. This Cr2 layer serves not only to prevent the Al line layer from being damaged during vacuum evaporation or sputtering but also to maintain the n.sup.+ a-Si:H property of the ohmic contact layer 28.
However, the constructed photoelectric conversion element and TFT of the conventional image sensor disadvantageously suffer from "field through", a phenomenon such that when a large voltage gate pulse .phi.Gi (i=1 to n) is applied from a gate signal line to each gate electrode 25, the potentials in the multilayer interconnections 13 and in the photoelectric conversion elements 11' are instantaneously increased by being pulled up by the gate pulse voltage.
The field through will be described in detail with reference to a circuit diagram shown in FIG. 10.
The circuit shown in FIG. 10 includes a photoelectric conversion element (PD) to which a predetermined bias voltage VB is applied. The photoelectric conversion element (PD) has a parasitic capacitor (CDi,j). A pulsed voltage (V.sub.GON -V.sub.GOFF) is applied to the gate electrode (G) of a thin film transistor (TFT) to turn on and off the gate, while a load capacitor (CL) is formed so as to store an electric charge generated at the photoelectric conversion element (PD) with the TFT as a switch. Potential variations at the load capacitor (CL) are read into a common line (COM).
The thin film transistor (TFT) includes overlap capacitors (CGD) and (CGS) existing between its gate electrode (G) and drain electrode (D) and between its gate electrode (G) and source electrode (S), respectively. Potentials at the drain electrode (D) and source electrode (S) are subjected to a variation called "field through" at the time the gate is turned on and off.
The potential variation (.DELTA.VD) caused by field through at the drain electrode (D) is determined by capacitances as expressed in the following way. EQU .DELTA.VD={CGD/(CGD+CDi,j)}.times.(V.sub.GON -V.sub.GOFF)
Further, the potential variation (.DELTA.VS) caused by field through at the source electrode (S) is determined by capacitances as expressed in the following way. EQU .DELTA.VS={CGS/(CGS+CL)}.times.(V.sub.GON -V.sub.GOFF)
The potential variation (.DELTA.VS) at the source electrode (S) is not so influential, because, the capacitance of the CL is sufficiently large. However, the potential variation (.DELTA.VD) at the drain electrode (D) affects transfer of electric charge because the small capacitance (CDi,j) causes the .DELTA.VD to become larger than the bias voltage VB. This causes current to flow reversely, resulting in incorrect transfer of the electric charge.
Further, the above-described conventional image sensor, in the course of its development for higher resolution and higher density from 300 spi (spot per inch) to 400 spi, must gradually down-size its photoelectric conversion element 11' and TFT. Under such circumstances, smaller parasitic capacitances (CDi,j) would suffer greatly from an instantaneous potential rise caused by field through, which would eventually lead to incorrect transfer of electric charge.